川中磨溪—高石梯地区栖霞组白云岩特征及成因机制

川中磨溪—高石梯地区栖霞组发育滩相白云岩，目前对滩相中差异白云石化机理不明确，导致难以预测优质白云岩储层的分布。在岩石学和地层特征研究基础上，通过分析不同类型白云岩的微量元素及碳、氧、锶同位素特征，结合颗粒滩类型和构造背景，系统研究了该区不同类型白云岩的成因机理及模式。结果显示:研究区栖霞组白云岩中白云石以细晶为主，中晶和粗晶次之，晶形多为半自形—自形。白云岩具有明显的残余颗粒结构，表明原始岩性为颗粒灰岩。细晶、细—中晶白云岩的阴极发光整体较暗，呈暗红色至红色，稀土元素配分模式与同期灰岩相似，白云岩的87Sr/86Sr比值大部分落于二叠纪海水的87Sr/86Sr比值范围之内，表明白云岩的成岩流体与海水沉积的灰岩具有同源性。白云岩的δ13C值（3.73‰~4.19‰）与同期灰岩δ13C值（3.61‰~4.93‰）相近，表明白云岩与灰岩具有一致的碳源。从灰岩到白云岩，Sr含量明显减少且Mn含量有所增加，说明灰岩经过一定的成岩作用被交代形成白云岩，该类白云岩为埋藏条件下地层中富Mg2+的流体交代孔隙型颗粒灰岩而成；中—粗晶白云岩的阴极发光呈红色，具明显环带特征，且具有高的Mn含量、低Sr/Ba比值及铕的正异常，87Sr/86Sr比值高于同期海水值，δ18O值在-8.06‰~-8.52‰，为颗粒灰岩在埋藏期受持续、充足的云化流体供给而成，较高的包裹体均一温度和δ18O值明显偏负均指示埋藏白云化作用过程还受到局部高温的影响。总体而言，埋藏白云岩化是该区白云岩的主要成因，地层中富镁的流体在压力和热对流的双重影响下进行迁移，促进白云岩化流体的运移，但局部地区鞍形白云石的形成遭受了后期不同程度的热液改造作用。

Characteristics and Genetic Mechanism of Qixia Formation Dolomite in Moxi-Gaoshiti Area,Central Sichuan Basin

The beach facies dolomite in the Qixia Formation is well developed in the Moxi-Gaoshiti area， central Sichuan Basin. At present， the mechanism of differential dolomitization in beach facies is still unclear， which hinders the prediction of high-quality dolomite reservoirs. This study combined an examination of the petrology and stratigraphy， grain type and tectonic background to determine the main controlling factors and genetic models of different types of dolomitization by analyzing the trace elements， carbon， oxygen and strontium isotope characteristics of different types of dolomites. The results show that， in the study area， the Qixia Formation dolomites are predominantly fine crystals， followed by medium crystals and coarse crystals. Most of the crystal forms are semi-automorphic to automorphic. The dolomite has an obvious residual grain structure， indicating that the original lithology was granular limestone. The cathodoluminescence of the fine-grained， and fine-to-medium crystalline dolomite is generally dark red to red in color. The distribution pattern of rare earth elements is similar to that of limestone of the same period. The 87Sr/86Sr ratios of the dolomites lie within the range of Permian seawater， indicating that the diagenetic fluid of the dolomite has the same homology as the limestone deposited in seawater. The δ13C value of dolomite （3.73‰?4.19‰） is similar to that of limestone （3.61‰?4.93‰） in the same period， indicating that the dolomite and limestone have the same carbon source. The Sr content decreases significantly from limestone to dolomite and Mn content increases， together indicating that the limestone was metasomatized to form dolomite after a particular mode of diagenesis: in this case， by replacement of porous granular limestone with Mg2+-rich fluid in the buried strata. The cathodoluminescence of medium-to-coarse crystalline dolomite is red， with obvious zonal characteristics， and has high Mn content， low Sr/Ba ratio and positive Eu anomaly. The 87Sr/86Sr ratios are higher than for seawater in the same period. The δ18O value is between -8.06‰ and -8.52‰. The higher homogenization temperature of the inclusions and the negative δ18O value together indicate that the buried dolomitization process was also affected by high local temperatures. This type of dolomite is formed from a continuous and sufficient supply of dolomitic fluid during burial. Overall， buried dolomitization was the main cause of the dolomite in this area. Mg2+-rich fluid in the formation migrated under the dual influences of pressure and thermal convection， which promoted the movement of dolomitizing fluid. However， in some areas， the presence of saddle-shaped dolomite indicates that it was subsequently subjected to various degrees of hydrothermal transformation in the later period.